Supports for Gas Separation Membranes. As is well known in the art, porous inorganic support materials for gas separation membranes can be classed into different categories: Metallic, ceramic, and glass. This invention is relevant to porous ceramic supports. Also, as well know in the art, support configurations can include at least the following: Tubular (with membrane on inner lumen or exterior surface), plate (for use generally in a stacked plate structure), hollow fiber (with membrane on the interior lumen surface, or more often the exterior surface), and multiple-passageway monolith structures (with the membrane coating applied to the passageway wall surfaces). This invention is applicable to all these membrane module configurations, but is especially suitable for high surface area multiple passageway monolith supports.
Ceramic Honeycomb Monoliths. Extruded ceramic honeycomb monoliths were initially developed as catalyst supports for automotive catalytic converters, environmental catalyst supports for fixed site installations, and diesel particulate filters. These monoliths have a multiplicity of passageways that extend from one end face to an opposing end face. The cell structure is formed by an extrusion process, with a cell density ranging from 9 to 1400 cells per square inch. For monoliths with circular cross sections, diameters can be as large as 12 inches, or greater. The length of such extruded monoliths can be over 6 feet, and is limited only by such factors as the available facilities for uniform drying and sintering. Numerous patents exist for such monoliths produced from cordierite (e.g., Lachman and Lewis in U.S. Pat. No. 3,885,997, and Frost and Holleran in U.S. Pat. No. 3,899,326) and silicon carbide (e.g., Stobbe in U.S. Pat. No. 5,195,319 and U.S. Pat. No. 5,497,620). However, ceramic monoliths with large diameters and lengths are generally difficult to manufacture using materials other than cordierite, silicon carbide or mullite, all of which have relatively low coefficients of thermal expansion (CTE).
Cordierite has frequently been used to produce monoliths because it has a CTE of ˜1×10−6/° C. and a microcracked structure. The low CTE and microcracked structure minimize differential mechanical stresses during the high temperature sintering and cool-down processes, thereby avoiding fracture.
Mullite has a somewhat higher CTE of ˜4.5×10−6/° C. However, its superior mechanical properties confer good thermal shock resistance, and the use of appropriate starting materials for mullite formation allows extruded bodies, including monoliths, to be sintered and cooled with a tolerable level of firing shrinkage that gives minimal formation of microcracks.
Silicon carbide monoliths, with a CTE of about 3.5-4×10−6/° C., have superior thermal and mechanical properties that permit their sintering and cooling without fracturing. First, the relatively high thermal conductivity of silicon carbide (˜5 W/m-K) relieves thermal gradients within the monolith. Second, the relatively high mechanical strength allows greater stress tolerance during sintering and cool-down.
In summary, present methods for manufacturing honeycomb-structured monoliths involve extrusion of suitably plastic batch materials through a die, followed by drying and sintering at an appropriate temperature to produce the final monolith. The choice of materials currently available for monolith fabrication is restricted to those that have a low CTE in order to prevent deformation and/or cracking of the monolith during sintering and subsequent cooling. At present, large honeycomb-structured monoliths are only commercially available in relatively low CTE materials, such as cordierite, mullite and silicon carbide.
Porous Ceramic Monoliths as Membrane Supports. Small-diameter ceramic monoliths are widely used as supports for inorganic membrane devices, and the patent art contains descriptions of monoliths produced from many different materials. Perhaps the earliest disclosure was in the French Patent Publication 2,061,933, filed Oct. 3, 1969 by the Commissariat a L'Energie Atomique, which describes a multi-channel α-alumina monolith as a support for an α-alumina ultrafiltration membrane. In 1978 Hoover and Roberts (U.S. Pat. No. 4,069,157) described the use of cordierite honeycomb monoliths as supports for dynamically formed membranes. In 1984, Gillot, et al., presented a paper “New Ceramic Filter Media for Cross-Flow Microfiltration and Ultrafiltration” (Filtra 1984 Conference, Oct. 2-4, 1984) that described the use of sintered α-alumina membranes deposited on sintered α-alumina monolith supports, closely related to the CEA French patent mentioned above. Abe, et al. (U.S. Pat. No. 4,894,160) disclosed the use of clay-bonded α-alumina as a honeycomb monolith support. In 1993 Faber and Hudgins described the use of titania as a monolith membrane support (U.S. Pat. No. 5,223,318). In 1995 Castillon and Laveniere (U.S. Pat. No. 5,415,775) disclosed the use of a mixture of titania/α-alumina monoliths as membrane supports. Grangeon and Lescoche describe metal oxide monolith supports containing titania in combination with other metal oxides, especially alumina (U.S. Pat. No. 5,607,586 and U.S. Pat. No. 5,824,220).
In general, porous α-alumina, configured in tubular and monolith structures, is the most common material used as a support for ceramic membranes, but only as small diameter elements. Such porous α-alumina materials are most commonly produced by sintering a monodisperse alumina at temperatures of 1600° C. to 1800° C. The use of clay, or other metal oxides, or fine α-alumina reactive binders can reduce the sintering temperature needed.
Large diameter honeycomb monoliths have been used for membrane supports for membrane devices. For example, the patents of Hoover and Roberts (U.S. Pat. No. 4,069,157), Hoover and Iler (U.S. Pat. No. 4,060,488), Goldsmith (U.S. Pat. No. 4,781,831, U.S. Pat. No. 5,009,781, and U.S. Pat. No. 5,108,601), Faber and Frost (U.S. Pat. No. 5,641,332), Yorita, et al., (U.S. Pat. No. 5,855,781), and Rajnik, et al. U.S. Pat. No. 6,077,436) disclose such devices.
The above large diameter monoliths used as membrane supports have all been conceptual designs or made from ceramic materials (cordierite, mullite or silicon carbide) that can be extruded, dried and sintered in large diameter parts while maintaining mechanical integrity. The decisive disadvantage of ceramics and ceramic composites formed by such a process is the high linear shrinkage that usually occurs between the green body and the final product, typically in the range of 5% to 15%. This shrinkage is problematic when trying to maintain the shape and dimension of a part. Shrinkage during sintering and cool-down can lead to the formation of cracks and other defects, up to and including the fracture of large parts.
Reaction Bonded Alumina Materials. Claussen has disclosed reaction-bonded alumina (RBAO) materials, in which α-alumina and related ceramic bodies are formed from precursor materials that react and retain “near net shape” during firing, i.e., undergo a negligible volume change in converting from the green (unfired) body to the sintered state (Claussen, U.S. Pat. No. 5,607,630). The RBAO process includes the use of powdered aluminum metal and ceramic grains in the batch formulations to form green bodies. During heating, the aluminum metal powder undergoes a volumetric expansion as a consequence of oxidation, and this volume increase offsets the normal shrinkage due to sintering of the ceramic grain constituents. The work of Claussen and those of several other groups active in the RBAO field focus on fabrication of near net shape bodies with low to negligible porosity. This low porosity and small pore size is achieved, in part, because the metal and ceramic grains used in the forming of the bodies are reduced to about 1 μm by aggressive attrition milling. Relatively high metal grain contents are also used, and this leads to filling of the pore volume during the oxidative expansion of the metal.
Claussen, et al., have also disclosed the fabrication of analogous near net shape ceramic bodies from other ceramic materials (Claussen, et al., in U.S. Pat. No. 5,326,519, U.S. Pat. No. 5,843,859, U.S. Pat. No. 6,025,065 and U.S. Pat. No. 6,051,277).
Variations on the chemistry of the process have also been reported, including the incorporation of ZrO2 in the RBAO body (Wu, et al., J. Am. Ceram. Soc., 76 (1993) 970), oxidation of a metallic Ba—Fe precursor to barium hexaferrite (Ward and Sandhage, J. Am. Ceram. Soc., 80 (1997) 1508), and the oxidation of aluminum with SiC to form mullite/alumina/SiC composites (Wu and Claussen, J. Am. Ceram. Soc., 77 (1994) 2898). The results of Wu, et al., are also included, in part, in the Claussen patents cited above. In all cases, however, the intention has been to form a dense ceramic part with essentially no open porosity.
More recently, the present Applicant has been awarded a patent by the United States Patent and Trademark Office for reaction bonded alumina filters and membrane supports (Bishop, et al., U.S. Pat. No. 9,695,967).
Inorganic Gas Separation Membranes. Inorganic gas separation membranes may be classed into at least three categories: Dense metallic membranes, dense ion transport oxide membranes, and microporous silica and zeolite membranes. Included in the category of metallic membranes are palladium, palladium-copper and palladium-silver alloys for hydrogen separations. For ion transport membranes, mixed conducting oxides are useful for separations of oxygen or hydrogen (the permeable species). For microporous oxide membranes, silica and a wide range of zeolite structures have been developed.
Many of these membrane materials have a relatively high coefficient of thermal expansion. Deposition of these membranes as a thin film onto a microporous support structure requires a reasonably close match between the CTE of the support and the CTE of the membrane layer. Table 1 provides ranges of CTEs for most inorganic gas separation membranes. The membranes relevant for the present invention are those that can be classed as metallic membranes and dense ion transport membranes, which have CTEs in excess of about 10×10−6/C.
TABLE 1Coefficients of Thermal Expansion of Gas Separation MembranesMembrane MaterialCTE, ×106/° C.Palladium and palladium alloys  12 to 16Dense oxide ion transport   9 to 20Microporous silica   1 to 3Zeolites −1 to 6
Deposition and use of the high-CTE membranes under conditions of temperature cycling requires use of support materials with similarly high CTE values. Porous α-alumina (CTE of ˜8.2×10−6/° C.) is at the lower CTE limit for a useful porous support material for palladium (and other metallic) membranes, as well as for low-CTE dense ITM membranes.
For ion transport membranes, supports can be metallic or ceramic. For ceramic supports, other than porous supports of the same mixed oxide composition of the ion transport membrane itself, different pure phase ceramic oxides have been used. For example, U.S. Pat. No. 5,599,383 (Dyer, et al.) and U.S. Pat. No. 5,681,383 (Taylor, et al.), both assigned to Air Products and Chemicals, Inc. disclose ceramic non-ion conducting supports including alumina, ceria, silica, magnesia, titania, stabilized zirconia, and mixtures thereof. Porous ceramic supports for ion transport membranes are also disclosed in U.S. Pat. No. 6,565,632 (van Hassel, et al.), assigned to Praxair Technology, Inc., including magnesia, alumina, ceria, and zirconia.
Supports used for palladium-based membranes are described by S. N. Paglieri and J. D. Way in “Innovations in Palladium Membrane Research”, in Separation and Purification Methods, 31(1), 1-169 (2002), specifically in pages 35-41. Support materials cited include ceramic, Vycor glass and stainless steel. Among ceramics named are alumina, titania, and stabilized zirconia. Roa, et al., in USP Application 2003/0190486 describe supports for palladium alloy membranes, including pure phase oxide ceramics (alumina, titania, and zirconia) and non-oxide ceramics (silicon carbide and silicon nitride) and sintered porous metals (stainless steel and nickel). The examples in the application employ alumina membrane coated alumina supports.
Support configurations for the above gas separation membranes have included primarily tubular elements and stacked plate devices, and in some instances multiple passageway monoliths and hollow fibers. The present invention is suitable for these support configurations, but is especially well suited for large diameter monolith supports. The invention has as its central feature the use of high-CTE, reaction-bonded ceramic materials as membrane supports, the materials preferably exhibiting nil or very low (<5% linear) shrinkage during the sintering of the green support structure body. This invention is similar to the Applicant's use of RBAO as a membrane support for membranes (Bishop et al., U.S. Pat. No. 6,695,967).
Alternative High CTE Ceramics for Membrane Supports. There are only a few oxide ceramics with relatively high CTEs that can be considered in reaction-bonded forms for practical, cost-effective production of membrane supports. Most notable are the single oxides of alumina (CTE of ˜8.2×10−6/° C.), titania (CTE of ˜8.8×10−6/° C.), stabilized zirconia (CTE of ˜10.0×10−6/° C.) and magnesia (CTE of ˜13.5×10−6/° C.), the mean CTEs given for the temperature range of 25-1,000° C. Additionally, there is the possibility of using high CTE ceramic compounds for membrane supports. These compounds include, but are not restricted to, magnesium orthosilicate (forsterite, Mg2SiO4) and magnesium aluminate (spinel, MgAlO2), which have mean 25-1,000° C. CTE values of ˜10.5×10−6/° C. and ˜8.5×10−6/° C., respectively. The present invention is based on the use of such ceramic oxides and/or compounds as porous supports, and employs a reaction-bonding mechanism during firing of the formed green support using elemental or other reactive binder precursors.